Exploring the space of quasi-isodynamic stellarators: a near-axis quasi-isodynamic database

Quasi-isodynamicity (QI) is a property of magnetic field configurations that supports plasma confinement—a prerequisite for achieving thermonuclear fusion—while offering several other advantageous features. These include a reduced bootstrap current and the potential suppression of certain microinstabilities. Since its inception in the 1990s, the design space of QI stellarators has been explored primarily through multi-objective numerical optimization of global equilibria. This approach has seen increased interest in recent years.

In this work, we investigate the QI stellarator landscape from a different perspective: using the near-axis expansion of the magnetic field. Building on recent theoretical developments, we construct a comprehensive database of stable, approximately QI magnetic configurations. These configurations span a range of field periods and controlled elongations. Our approach is grounded in a novel geometric construction method that allows for enhanced control and systematic exploration of configuration space.

To evaluate the physical quality of each configuration, we define a broad set of figures of merit within the near-axis framework. These include effective ripple, sensitivity of the Shafranov shift to changes in plasma beta, the percentage of maximum-J trapped particles, and the Rosenbluth-Hinton residual, among others. This enables an exhaustive, thorough and quantitative characterization of the database.

Our database offers a valuable window into the space of QI stellarators. It forms a foundation for identifying configurations that optimize specific physical properties that may serve as promising initial candidates for further global optimization. Furthermore, statistical analysis and modern machine learning techniques may be employed to uncover hidden correlations, identify key descriptors, and guide the generation of novel optimised configurations. With this work we initiate a long term program to complete a systematic exploration of QI stellarator design space.